Polyacrylamide crosslinked with Cr(III) is a common blocking agent that is injected into oil reservoirs to shut off the water flow to features such as fractures and high-permeability rock to improve the volumetric sweep efficiency during water flooding. This technique is limited to situations where enough gelant can be injected to fill the high-permeability zones before gelation occurs. Cr(III) may be encapsulated with a high entrapment efficiency in self-assembling polyelectrolyte complex nanoparticles with effective diameters of about 100-200 nm and formed by the mixture of polyethylenimine and dextran sulfate in nonstoichiometric amounts. The electrostatic interaction between the polyelectrolytes and the Cr(III) is reversible, but diffusion out of the nanoparticles is retarded, and this results in a delay in gelation from minutes or hours to several days in deionized water. Produced and injection waters often contain salts at concentrations comparable to seawater. The effects of these salts were investigated, and we found that the delay in gelation was further extended to as much as 35 days at 40 degrees C and 35 h at 80 degrees C in the presence of divalent ions at seawater concentrations, either alone or in afield brine or seawater; this greatly increases their potential utility. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 1008-1014, 2010